It is generally known to spectrally sensitize photographic silver halide emulsions so as to extend their sensitivity range. For instance, it is known to spectrally sensitize photographic silver halide emulsions with trinuclear cyanine dyes comprising a benzothiazole nucleus linked by a dimethine chain to a thiazolidone nucleus carrying a carboxymethyl group, which thiazolidone nucleus is linked directly to a second thiazolidone nucleus. An example of such dyes is the triethylamine salt of 3-carboxymethyl-5-[(3-ethyl-2-benzothiazolinylidene) -ethylidene]-2-(3-allyl-4-oxo-2-thio-5-thiazolidinylidene)-4-thiazolidone, which has been disclosed in SU-A 1,164,248.
Spectral sensitization for the red colour wavelength range is required to impact a desired red sensitivity to photographic silver halide emulsion materials for use in widely different application fields e.g. to photographic materials for camera exposure, positive photographic materials for contact papers or enlarging papers, photographic printing materials, and photographic materials for recording by means of light sources emitting in the red colour wavelength range such as e.g. diode light, laser light, and cathode ray tubes (CRT).
In recent years electronic colour separation scanners equipped with a laser light source have been used in many cases in the printing field. In particular a helium/neon laser having a power wavelength of 632.8 nm is often used, e.g. for the formation of dot images.
It is known to spectrally sensitize for this wavelength e.g. with the aid of sensitizing dyes described in U.S. Pat. No. 4,717,650.
Many of the dyes known for spectrally sensitizing photographic silver halide emulsions for the red wavelength range of 600 to 690 nm have the disadvantage of having a low solubility in water so that they usually have to be dispersed in the emulsions or added thereto in the form of an alcoholic solution. Consequently, they are not removed adequately during the processing. As a result, a considerable residual magenta to blue hue is left in the dried material. This considerable residual hue is disturbing in photographic materials and especially so in photographic materials comprising a white paper support.
There thus remains a need to find a method for forming silver images that are free or almost free of residual colour stain in wet-processable photographic silver halide materials, said materials having been spectrally sensitized with dyes that in addition to providing a desired spectral sensitivity range of from 600 to 690 nm and a satisfactory speed and gradation to the silver halide emulsion, are sufficiently soluble in water so that after development and fixing of the photo-exposed silver halide emulsion they are rinsed away for the greater part.